Process for forming a nonwoven web

ABSTRACT

A process for spreading, electrostatically charging, and forwarding a fibrous web concomitantly formed with a vapor blast. The web is charged by passing it through a highly ionized zone created by a corona discharge between an ion gun and a target plate. The electrical potential between the ion gun and target plate causes current to flow which is sufficient to deposit a charge on the web which is preferably 75 to 100 percent of the maximum sustainable charge, but low enough to avoid loss of web charge through secondary corona discharge between the target plate and the web.

United States Patent Hollberg et al. Sept. 5, 1972 541 PROCESS FORFORMING A [56] References Cited NONWOVEN WEB UNITED STATES PATENTS [72]Inventors: Herbert John Hollberg, Richmond,

v John Edward Owens, Hockes- 3,081,519 3/ 1963 Blades et al. ..264/5 i Dl. 2,810,426 10/1957 Till et al. ..264/24 2,048,651 7/1936 Norton..264/D[G. 75 [731 Asslgnw Nemours and 2,336,745 10/1943 Manning..264/DIG. 75

pany, Wilmington, Del. 22 Filed; June 10, 7 Primary Examiner-Robert F.White Assistant Examiner-W. E. Hoag PP Attorney-Howard P. West, Jr.

Related US. Application Data [57] ABSTRACT v [63] Continuation of Ser.No. 735 889 June 10 A process for. spreading, electrostatlcallycharging, 1968 abandoned whlch a a and forwarding a fibrous webconcomitantly formed part of June 4 with a vapor blast. The web ischarged by passing it No. 3,387,326. 7 through a highly ionized zonecreated by a corona discharge between an ion gun and a target plate. The

[52] U.S.Cl. ..264/24, 28/72.12, 28/76, electrical potential between theion gun and target 264/53264/ 121 plate causes current to flow which issufficient to. [51] ll'lt. Cl. ..D04h 3/03 I deposit a charge on the webwhich s preferably 75 to Fleld of Search 53, percent of the maximumustainable charge but 18/8 E, 8 W, 8 B; 28/72 .12-, 76

low enough to "avoid loss of web charge through secondary coronadischarge between the'target plate and the web.

5 Claims, 8 Drawing Figures PA'IENTEMEP'BM O 3.689.808

snztnuia I ll/l/ll/l/l/fl/l ll/ I TO SOLVENT RECOVERY POLYMER SOLUTIONSUPPLY HERBERT JOHN HOLLBER'G JOHN EDWARD OWENS BY 1 Mai ATTORNEYPATENTEDSEP T972 3. 689 .608 SHEET 3 or 5 0 I20 I40 I60 I80 200 220TARGET PLATE CURRENT INVENTORS HERBERT JOHN HOLLBERG JOHN EDWARD OWENSBY NW ATTORNEY PROCESS FOR FORMING A NONWOVEN WEB CROSS REFERENCES TORELATED APPLICATIONS This application is a continuation of U.S.application Ser. No. 735,889, filed June 10, 1968 and now abandoned,which is a continuation-in-part of U.S. application Ser. No. 372,623,filed June 4, 1964 now U.S. Pat. No. 3,387,326.

BACKGROUND OF THE INVENTION This invention concerns a novel and usefulprocess for charging fibrous webs in an electrostatic field anddepositing the webs uniformly in overlapping layers on a moving surfaceto form a nonwoven sheet.

The process described and claimed herein is particularly useful incharging webs of a continuous fibrillated strand described in U.S. Pat.No. 3,081,519 to Blades and White. This web is prepared by flashextrusion of a solution of crystallizable polymer. In the flashextrusion process the strand is formed by extruding a homogeneoussolution of a fiber-forming polymer dissolved in a liquid. The solution,at a temperature above the normal boiling point of the solvent and atautogeneous or greater pressure, is extruded into a medium of lowertemperature and substantially lower pressure. The vaporizing liquidwithin the extrudate forms bubbles, breaks through confining walls, andcools the extrudate, causing solidification of the polymer.

The resulting fibrous web is a multifibrous yarn-like strand having aninternal fine structure or morphology which may be characterized as a3-dimensional integral plexus consisting of a multitude of essentiallylongitudinally extended interconnecting, random-length fibrousfilaments, hereafter referred to as film-fibrils. These film-fibrilshave the form of thin ribbons with an average thickness less than about4 microns. The filmfibril elements often found as aggregates,intermittently unite and separate at irregular intervals calledtiepoints" in various places throughout the width, length, and thicknessof the strand to form an integral 3-dimensional plexus. The film-fibrilsare often rolled or folded about the principal film-fibril axis, givingthe appearance of a fibrous material when examined withoutmagnification. The strand comprising a 3-dimensional network offilm-fibril elements is referred to as a plexifilament. Theplexifilaments are unitary, i.e. the strands are one continuous piece ofpolymer, and the elements which constitute the strand areinterconnected. They can be produced in essentially endless lengths indeniers as low as or as high as 100,000 or even higher.

The plexifilament of Blades and White may be collected in the form of anonwoven fibrous sheet and may be consolidated by cold or hotcalendering to provide useful sheet products. These products and theprocess for making them are described in Steuber U.S. Pat. No.3,169,899. This patent describes an electrostatic device for promotingattraction of the strand to a collecting belt. The device is verysatisfactory for preparing nonwoven fibrous sheets with exceptionalstrength. However, in further developing the process, it has becomeevident that improvements are needed to provide a high degree ofdispersion and uniformity in sheets destined for certain uses. Theseimprovements are needed particularly when the sheet is to be used inprinting papers, book covers and wall coverings. It has been discoveredthat the requirements for aerodynamic stability of the fine fibrilnetwork and'the requirements for uniform electrostatic charging aresomewhat in opposition to each other. These requirements must thereforebe carefully matched for production of uniform sheets.

SUMMARY OF THE INVENTION The purpose of the present invention is toprovide an improved aerodynamic and electrostatic process for spreadingand charging a plexifilament strand and for depositing the strand in theform of a nonwoven sheet with a high degree of dispersion anduniformity.

In the process of the invention the flash spinning, spreading, anddepositing operations are conducted in a closed chamber to provide auniform high dielectric atmosphere. A freshly spun plexifilament strandand the accompanying expanding solvent gas are directed from thespinneret to a spreading zone created by a baffle or other confiningsurface whereby the plexifilament strand is opened into a wideconfiguration. The spread strand is passed in a path of advance directlyfrom the spreading means into a highly ionized zone created by coronadischarge in the atmosphere between an ion gun and a flat target plate.The electric potential between the ion gun and target plate issufficient to generate a current flow to deposit a charge on the spreadstrand which is preferably to percent of the maximum sustainable peakcharge, but is low enough to avoid disruptive spark discharge orsecondary corona discharge between the thin trailing edge of the targetplate and the strand. The target plate is placed immediately adjacent tothe mechanical spreading means in such manner that the vapor blast fromthe spinneret guides the web to provide brushing contact with thetarget. The surface of the target plate is of planar construction,particularly in the area just upstream of the trailing edge. The targetplate terminates in a thin trailing edge to provide uniform but minimumaerodynamic turbulence at this point during operation. The ion gun is astructure supporting a plurality of charging needles disposed across thepath of advance. The gun is placed opposite the target and mounted in amanner that permits circulation of vapor around it, since duringoperation the confined gases tend to flow toward the path of advanceover the top of the gun. Preferably the face and the top of the gunhousing are smooth and shaped to minimize aerodynamic turbulence. Theneedles are aimed at points which are uniformly spaced from the trailingedge of the target plate by a technique described hereinafter. Thespread and charged fibrous web is then deposited on a continuouslymoving surface, electrically discharged and collected by conventionalmeans such as windup in a roll.

BRIEF DESCRIPTION OF THE DRAWINGS of another apparatus embodiment usefulin the practice of the invention.

FIG. 3 is an elevation showing the relative positions of the ion gun,rotary baffle and target plate of FIG. 2, the spinneret nozzle beingremoved for clarity.

FIG. 4 is an enlarged partially sectioned fragmentary view of FIG. 2,showing the relationship of the conducting needles to the targetelectrode.

FIG. 5 is a partial cross-sectional schematic elevation illustrating ashrouded spinning orifice useful in separating the plexifilament asspun.

FIG. 6 is an enlarged partially sectioned front elevation of theshrouded opening orifice and target plate of FIG. 5.

FIG. 7 is a curve wherein web charge, percent peak web charge, and beltcurrent are plotted as ordinate vs. ion gun current as abscissa.

FIG. 8 is a series of curves wherein web charge is plotted as ordinatevs. target plate current as abscissa.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS Referring to FIG. 1,a spinneret device 10, connected to a source of polymer dissolved in anorganic solvent is shown. Polymer solution 12 under pressure is fedthrough spinning orifice 14 into web forming chamber 16. The extrudatefrom spinning orifice 14 is a plexifilament 7. Due to the pressure dropat spinning orifice 14 vaporization of solvent creates a vapor blastwhich, by virtue of impingement upon baffle 18 concomitantly withplexifilament 7, generally follows the path of advance of theplexifilament 7 from spinning orifice 14 to collecting surface 9,thereby creating a flow pattern within chamber 16 as indicated by thearrows. Baffle 18 is oscillatably mounted and is powered to oscillate bymeans not shown. While oscillation of the baffle is not essential, it ispreferred for the preparation of wide sheets.

As shown target plate 20 and ion gun 22 are disposed on opposite sidesof the path of advance of the plexifilament web 7 and downstream fromthe web forming and mechanical separating devices. Target plate 20 isconnected to ground by wire 24 and microarnmeter 26 which indicatestarget plate current. Ion gun 22 contains multiple needles 25, one ofwhich is shown in FIG. 1. Each needle 25 of ion gun 22 is connected to anegative D.C. source 35 through resistor 19. Each of the resistors isconnected to the source of power through conductor 21. Millameter 23serves to measure ion gun current. A negative D.C. source in the rangeof from 45 to 70 kilovolts may be used. Target plate 20 is so disposedthat the vapor blast originating at spinning orifice l4 and the air flowpattern in Chamber 16 carries plexifilament web 7 in brushing contactwith its charging surface. After passing through an ionized chargingzone created by a corona discharge between ion gun 22 and target plate20, the charged plexifilament web 7 is deposited on collecting surface9. The surface illustrated is a continuous belt forwarded by drive rolls36. The belt is given an opposite charge to that imposed on web 7 bymeans of D.C. source 37 which is connected to the collecting apparatusthrough milliameter 29 and lead 27. Due to the opposite polarity betweenweb 7 and surface'9 the web in its arranged condition clings to thesurface as sheet 38 with sufficient force to overcome the disruptiveinfluences of whatever vapor blast may reach this area. Surface 9carries sheet 38 past compacting roll 44 and feeds the sheet out ofchamber 16 through port 39 where it is collected on windup roll 42.Flexible elements 40 across port 39 assist in the retention of vaporwithin chamber 16. Roller seals or labyrinth seals may also be used. Aconventionalsolvent recovery unit 43 may be beneficially employed toimprove economic operation.

Alternate apparatus embodiments useful in the practice of the inventionare shown schematically in FIGS. 2-6. Referring to FIG. 2, the extrudatefrom orifice 14 is carried around the curved surface of a lobed baffle18 into brushing contact with the surface of an annular target electrode20'. Baffle 18' is continuously rotated to impart oscillatory movementto the network of film fibril material as it is deflected from the lobedsurface. Annular target electrode 20' is coupled, for rotary movementabout baffle 18' by means of ring 50 and pinion gear 52 attached todriven shaft 54. Target electrode 20 is connected to lead 24 through acontacting carbon brush 56. Ion gun 22' is U-shaped and is connected toa negative D.C. source through lead 24. FIG. 3 shows the arrangement ofU-shaped ion gun 22 opposite annular target electrode 20' with thebaffle 18 centered within the electrode. Needles 25 are arranged in thelower tubular portion of the ion gun 22 such that the axes of theneedles are generally perpendicular to the surface of target electrode20 (FIG. 4).

An alternative mechanical spreading arrangement is illustrated in FIGS.5 and 6 where spinning orifice 14 is surrounded by a shroud 15 having astepped slot 17 therethrough. The plexifilament on extrusion tends toopen and follow the stepped contour of slot 17. The extrudate can beimpinged on a fixed or moving baffle or directed along the path ofadvance without baffling (as shown) when the shrouded orifice isemployed to spread the web. Other shapes for slot 17 may be successfullyemployed as for example, a bell or a conical shape. Positioning of iongun 22 is important to obtain maximum charging efficiency and also toavoid web bunching and flicking which are detrimental to sheetuniformity. Bunching is a small pileup which occurs when a web passingdown a target plate is slowed by pinning forces. Flicking occurs whenfast moving web hits this bundle and flips it away from the targetplate, sometimes resulting in hangup on the needle point 25 and alwaysdischarging the web unevenly. Thus, although a short distance betweenneedlepoint 25 and target plate 20 provides a relatively low voltagerequirement to produce a given target plate current, close spacing canonly be tolerated if web flicking and bunching is held to a minimum. Theproblems are particularly acute in the production of sheets fromplexifilamentary structures due to the fluffy nature of theplexifilament which makes it particularly susceptible to irregularitiescaused by non-uniform aerodynamic or electrostatic patterns. Use of abaffle or spinneret shroud helps to spread and thereby dissipate thevapor blast that flashes from the spinneret. A high velocity vaporstream at the collecting surface otherwise disarranges deposited websand causes them to roll. Thus a smooth pattern of vapor flow withinchamber 16 is important to assist in the orderly forwarding ofplexifilament 7 along its path of advance from spinning orifice 14 tocollecting surface 9 while avoiding interference with the plexifilamentat the collecting surface. Equipment shapes to promote these aerodynamicdesirata are important for efficient and high speed operation. Forinstance the targets 20, 20' shown in FIGS. 1 and 4 terminate in a thinedge, and the target surface is planar just upstream of the edge, such ashape being important to promote smooth vapor flow despite theelectrical discharge known to be associated with sharp edges. A thinlayer 58 of epoxy resin at the outer edge of the target electrode 20' asillustrated in FIG. 4 is useful in reducing secondary ionization at theedge of the target electrode by eliminating a sharp conductive edge. Useof a resistor 19 in series with each needle has been found to provideneedle-to-needle current uniformity important in the production ofuniform sheet products especially when operating at a low current perneedle. Operation below about microamperes per needle is desirable whenusing an ion gun with resistors separating each needle from the currentsource. With needle separation of about three-eighths inches in an iongun of this type, between about 6 and 8 microamperes per needle is usedfor depositing a linear polyethylene plexifilarnent. The ion gun with aresistor in series with each needle provides a high impedance circuit toeach needle point so that normal fluctuations in the effective dynamicresistance of corona discharge have little effect on emitted current. Ina typical ion gun/target configuration the effective dynamic resistanceof corona discharge is about 60 megohms, whereas the resistance 19placed in series with each point is typically 600 megohms. Use of theresistors makes the needle-toneedle current variations much lesssensitive to such factors as point/target spacing, point wear, pointcontamination, and point-to-point spacing.

The position of needles 25 with reference to target plate 20 isimportant for efficient operation. It will be apparent that theclearance between the needle points and plate 20 should be as small asefficient operation will permit. Generally a clearance of from about 0.4to about 2 inches (1 to 5 cm.) is satisfactory although this will varywith the design and capacity of the particular equipment. It has beenfound convenient in adjusting positioning of gun 22 opposite to targetplate 20 to create a carbon black deposit on target plate 20 by sprayingpowdered carbon black into the operating area between the plate and thegun. An oval pattern is outlined by carbon deposits opposite each needleindicating the area of electrostatic influence of each needle under theparticular conditions employed. Such a pattern of carbon deposit 13 isshown in FIG. 6. The patterns laid down by single points are centeredthe same distance apart as the needles, are oval shaped, and have aheight of about 2.5 cm. and a width of about 0.6 cm. Smoothest operationof the equipment with uniform laydown occurs when the above-mentionedtest patterns are centered at a distance between onehalf in. (1.3 cm.)and three-fourths in. (1.9 cm.) from the bottom edge of the target.Placement of the ion gun at a point further upstream results in pinningor clinging of the web to the target plate because of fieldconcentration between the already charged fiber and the thin edge. Thisresults in bunching for an instant, an uneven discharge across the webwidth, and a falling free of the bunched web to give a nonuniform sheet.In addition, when the gun is aimed further upstream on the target plate,the web charge curve is very abrupt as will be demonstrated hereinafter,making the process more difficult to control. On the other hand if theion gun is aimed too near the trailing edge of the target plate,secondary ionization will develop at the edge of the target plateproviding positively charged ions which will discharge the web unevenly.The web will then collapse and give a ropey strand which in turn gives anonuniform sheet. In addition the discharged web will not pin well tothe belt because it has lost most of its charge. In general the targetplate must be of such dimensions that in cooperation with the vaporblast, it will guide the mechanically opened web into the electrostaticcharging zone, which zone must be sufficiently removed from possibleinterfering grounded structures such as spinneret 10 or baffle 18 sothat shorting out of the gun does not occur.

In general, in providing field-assisted laydown of plexifilament 7,three methods may be used to produce strong electrostatic pinning forceson the charged fibers:

1. Use of a conductive laydown roll or belt, insulated from ground andraised to a high potential (e.g., KV).

2. Use of a semiconductive laydown belt, in contact with a stationaryelectrode to which a high potential is connected.

3. Use of a porous woven belt of insulating material in contact with anelectrode to which a high potential is connected.

The two critical electrostatic requirements placed upon the laydownsurface are:

1. That an intense electric field can emanate from or be transmittedthrough the laydown surface toward the approaching fibers.

2. That the current produced by neutralization of charged fibers at thelaydown surface have a path to ground. In the case of method 3 above,the path is through the interstices of the woven belt, wherein vapor ismade conductive as a result of ionization occurring at the laydownsurface.

Two requirements for effective charging are a high density of ions of asingle polarity and a high electric field intensity in the vicinity ofthe fibers. For a given ion gun-target plate geometry, ion density isdetermined primarily by the value of corona current. Web chargingresults from impingement of ions onto the fibers as the ions move towardthe grounded target electrode. Approximately 10-15 percent of the totalcorona current is carried away as charge on the fibers because theprojected web area is small compared to the cross-section of the ionstream. To place the highest charge on a given mass of web it isnecessary to have the fibers close to the target electrode so that thefield force lines from the charges on the fiber are directedpreferentially toward ground, This directionality of field force ineffect reduces the field force component that tends to repel additionalions and prevent them from depositing on the fiber surface. Very littlecharge is lost through web contact with the target electrode surface.The negative ions impinge on the side of the fibers away from ground,and conductivity of the fibers is too low to leak much of the charge tothe target. In addition, it is probable that a thin layer of solventvapor lubricates the target, keeping most of the fibers out of directcontact with it.

The optimum web charge for a given combination of apparatus, polymer andsolvent may be determined by considering the relationship of targetplate current versus web charge. Three relationships are shown in FIG. 8for two different ion guns, the equipment being otherwise identical,where one gun is operated during two different polymer flow rates. Ineach instance the clearance between the points of needles 25 and targetplate is 1.5 inches. Other dimensional and operational variations foreach of curves A, B and C are listed in Table I below, where polymerflow rate is in pounds per hour and target aim is the distance in inchesfrom a point on target plate 20 directly opposite the point of needle tothe bottom edge of target plate 20.

The necessary data are obtained from spinning experiments wherein theelectric potential (in kilovolts) between ion gun and neutral ground isincreased incrementally, and the target plate current (observed at 26 inmicroamperes) and the web charge (in microcoulombs/gram) are determinedand recorded. Web charges are determined by collecting the web after itleaves the target plate and before it reaches the collecting belt for agiven period of time in a Faraday pail. The potential relative to groundto which the pail rises during the collecting period is measured by anelectrostatic voltmeter (e.g. Rawson type 518, Rawson ElectricalInstruments Co., Cambridge, Mass). A high quality capacitor is connectedacross the input terminals of the voltmeter to provide an on-scaledeflection of the voltmeter corresponding to the accumulated charge.This value of capacitance is normally substantially larger than thetotal other capacitances in the metering circuit. From the well knownrelationship between voltage, charge and capacitance, the chargecollected per gram is calculated as follows:

where:

q =web charge level, microcoulombs/gm C capacitance, microfarads V=indicated voltage, volts t =sampling time, seconds W=throughput of theweb gm/sec From a consideration of the curves it will be noted thatincreasing web charge is obtained with increasing target plate current(obtained by increasing potential) until a peak is reached. Thereaftersecondary ionization becomes significant and it becomes thenincreasingly difficult to retain a charge on the web. Secondaryionization is characterized by a glow discharge at the trailing edge ofthe target plate between the target plate and film-fibrils as they leavethe target plate. For uniform web formation it is preferred to operateat a voltage between ion gun and neutral ground that will provide a webcharge between about percent and percent of peak value undernon-secondary ionization conditions. The sharp peak of curve A istypical of the condition wherein needles 25 are aimed too far upstreamfrom the edge of target plate 20. Under these conditions it isrelatively difficult to maintain a constant charge on successiveportions'of the web and across the width of the web. Much moresatisfactory control is obtained in situations such as those shown incurves B and C. In all of the curves A, B and C increasing the targetplate current above the peak charge level for the web has detrimentaleffects in that the web tends to pin or cling to the target plateresulting in bunching and flicking which are detrimental to the sheetuniformity. This occurs because of secondary ionization which causesnon-uniform loss of charge, uneven collapse, and uneven laydown, theplexifilament 7 tending to roll during laydown if it is not properlyelectrostatically held to the collecting belt. This causes a sheet ofpoor uniformity and rope-like fiber bundles appear in the sheet.

It is to be noted that very high charge levels are obtained onplexifilamentary webs compared to solid fibers melt-spun at the samecorona current level. For example, one can obtain a charge of 5microcoulombs/gram on linear polyethylene plexifilament with onlymicroamperes of current with a 25 point gun (6 microamps per coronapoint). Typical melt spun fibers such as those described in US. Pat. No.3,341,394 to Kinney require a current of 50 to 75 microamperes/point toobtain charges at this level.

The process and apparatus of this invention are particularly useful forflash-spinning in a solvent laden atmosphere. It is desirable to spininto an atmosphere containing less than 30 percent air (more than 70percent gaseous solvent). Spinning of this type must be done withpolymer/solvent combinations that separate rapidly on cooling. It isthen possible to spin into a closed chamber and have adequatesolidification and crystallization of the fiber structure. Thus, asolution of linear polyethylene and trichlorofluoromethane (Freon-ll ofDu Pont) may be spun into a closed chamber, whereupon the web is spreadby a baffle or shroud, is charged electrostatically, and is deposited ona moving belt. The gaseous solvent may then be recovered by compressionand condensation without difficulty. In the open ventilated cellspreviously used this would have been much more difficult because of thelarge amount of air present.

EXAMPLE A plexifilament of linear polyethylene was spun from a solutioncontaining 12.5 percent-i 0.3 percent linear polyethylene by weight, and87.5 percent i 0.3 percent trichlorofluoromethane (Freon-l l and 1,750ppm of an antioxidant (Irganox No. 1010). The solution was pumpedcontinuously through a pipeline to a single spinneret pack. The solutionwas delivered to the spinneret pack at a temperature above the boilingpoint and at a pressure close to the critical pressure of the solvent.The solution was spun through a spinneret of the type shown in FIGS. 2,3 and 4 at a rate equivalent to 35.0 35.8 lbs/hour of polymer. As thesolution passed in a horizontal direction through orifice 14' into theatmosphere of the enclosure, the solvent evaporated and a plexifilamentwas formed. This plexifilament was spread and directed downward into avertical path by passage over the rotary baffle 18. At the same time thecombined action of the expanding solvent gas and the curved surface ofthe baffle spread the plexifilament into a wide web. This web thentraversed annular target plate 20. The target plate outer diameter was19.0 cm. and the inner diameter was 14.0 cm. The outer trailing edge ofthe target plate comprised a bead of non conductive epoxy resin set intothe rim of the target plate as shown in FIG. 4. The bead width in theplane of the target face was 0.32 cm. The web was directed downwardacross the trailing edge 58 and continued toward a continuously movingcollecting belt of wire mesh traveling at 60 ft./min.

The spread web was exposed to the ionized atmosphere between negativelycharged ion gun 22 and target plate 20' during passage and therebycollected a negative charge. The ion gun was a U-shaped device having 24needles spaced 0.95 cm. apart. In this experiment the needles wereattached directly to a common power source and no resistors were used inthe needle connections. The curved portion of the U-shaped ion gun wassemi-circular and concentric with the annular target plate. The needlepoints were located opposite the target plate 1.43 cm. from the outeredge (including 0.32 cm of epoxy rim and 1.11 cm. of metal). The needlepoints were 1.59 cm. from the target plate surface. The collecting beltwas either positively charged or was neutral (grounded), depending uponthe particular test items. A number of test conditions were studied andare recorded in Table II.

The spinneret pack included a letdown chamber and a letdown orificeupstream of the final orifice 14. The letdown orifice was 0.035 inch(0.889 mm.) in diameter and passed through a land 0.025 inch thick(0.635 mm.). The letdown chamber volume was 24 emf. The final orificewas 0.030 inch (0.762 mm.) in diameter and the land for the finalorifice was 0.25 inch (0.635 mm). The solution was provided to theletdown orifice at a temperature of l85.5C. and a pressure of 1,7501,800 psig (123.5 to 127.0 kg./cm. It passed then through the letdownorifice into the letdown chamber, which was maintained at a pressure of1,050 psig. Finally, the solution passed from the spinneret orifice intoa cylindrical tunnel (not shown) in the conical end of the spinneretpack. The tunnel was concentric with the orifice hole. The tunneldiameter was 0.188 inch (0.478 cm.) and the length was 0.188 inch (0.478cm.). The spinneret pack was located with the orifice 13 inches (33.0cm.) above the belt. The bottom of the target plate was 2.7 inches (6.86cm.) below the orifice.

During the spinning operation the Freon concentration in the closedchamber surrounding the spinneret pack was about 93 to 96 percent byvolume, the remainder being mostly air. The charged web was collected onthe moving belt and was consolidated by passage under a roll at the endof the belt which provided a pressure of about 34 lbs/linear inch (6.1kg./cm.). The roll diameter was 9.65 inches (24.5 cm.). The rolltemperature was about 55C.

Baffle 18 as shown in FIG. 3 contains three lobed fillet portions. Asthe baffle turned about its axis, these lobed portions diverted theplexifilaments either to the left or right of the center line, providingan oscillating motion in the strand. The fibrous strand was thereforedeposited in oscillating fashion on the belt in multidirectionalover-lapping layers. The belt was forwarded at a speed of ft./min. andthe baffle turned at a speed of 1,400 revolutions/min; consequentlyseveral multidirectional layers were collected at each point along thelength of the sheet at its center of width. In a commercial operationsheet of much greater width may be obtained by depositing overlappinglayers of plexifilaments from many spinnerets on a single belt.

The annular target plate 20' was adapted to rotate at a speed of 2.3revolutions/min. about an axis concentric with the rotating baffle axis.The target plate was provided with a wicking device (not shown) whichcoated the surface of the target with Zelec U.N. lubricant, a conductiveliquid which was beneficial for maintaining a uniform conductive path toground during the test. The target plate was grounded through conductor24. A microammeter was provided between a power pack and U-shaped iongun 22. In addition, a microammeter was provided between the conductivecollecting belt and either ground potential or a positive DC source.

In this series of experiments the spinneret pack of FIGS. 2 to 4 waslocated over a moving belt similar to the one shown in FIG. 1. The beltcurrent measured by microammeter 29 was used to indicate extent of fibercharging. Some of the test items were run with no applied voltage on thebelt and others were run with 15 kilovolts positive potential applied(oppositely charged relative to fiber). In either case the beltmechanism was electrically insulated from ground except for the paththrough microammeter 29 or through both microammeter 29 and positivedirect current source 37. It has been found that substantially all ofthe current flowing from the ion gun is collected by either the targetplate or by the collecting belt; thus where 1,, equals the ion guncurrent, 1,, equals target plate current to ground, and 1,, equals beltcurrent to ground. The charge on the fibers was calculated from the beltcurrent I,, and the polymer flow rate W by means of the equation:

wherein 1,, is the belt current in microamperes, Wis the weight in gramsof fiber passing between the ion gun and target plate per second, Q isthe charge expressed in microcoulombs per gram.

The data from this series of experiments are reported in Table II. Thevarious items in Table II are listed in order of increasing ion guncurrent. The suffix letters A to V indicate the chronologic order, Abeing first and V being last. It has been found that this order isimportant in cases where the target plate accidentally becomes coatedwith polymer residues. These residues may form a hard varnish which inturn tends to change the conductivity of the target plate or tends topromote the formation of high current densities in pin-point areas orcavities on the target plate. This in turn causes spark discharge, lowweb charge level, and low belt current. A comparison of Items 10A and11V shows that this condition was avoided. In addition the data in TableII indicate that the current from collecting belt to ground wassubstantially the same with or without belt potential applied. Compare,for example A with 9B or 8U with 11V.

Now considering the effect of various ion gun current levels on qualityof the deposited sheet, Table II shows the width of the deposited swathand the range in width with ion gun currents from 100 up to about 500microamperes. The sheet width was measured outside the spinningenclosure over a period of time and the maximum and minimum values wererecorded. The width of the sheet included all of the deposited webregardless of thickness. In Table II no range is shown when the widthvaries less than 0.5 inch.

The basis weight averages for the sheets reported in Table II wereobtained from circular samples eachl inch (2.54 cm.) in diameter. Thesamples were cut from approximately the center of the collected sheetwidth. In each case the basis weight average was determined from 120samples in three rows of 40, the samples in each row being taken 6inches (15.24 cm.) apart along the length of the sheet. The rows were 3inches (7.62 cm.) apart in the cross-sheet direction.

The basis weight uniformity was established by calculation of standarddeviation 0' defined by the formu- 2 (X -X) N: n

where Y average basis weight in oz./yd.

X individual basis weight in oz./yd.

n number of samples and Z (X 1?? the summation of the square of thedifferences between X and X.

The data recorded in Table II showthat the standard deviations were at aminimum with Items 3E through 12I (ion gun current 200 to 325microamperes). In this same range the maximum sustainable swath widthwas between 18 and 21 inches (45.8 and 53.2 cm.). In multiple spinneretoperations the highest possible swath widths are most desirable sinceless equipment is required. For this reason in this example Items 4Qthrough 121 are especiallypreferred (ion gun currents 225 to 325microamperes and swath widths 19.5 to 21 inches (49.6 to 53.2 cm.). Inthe discussion of FIG. 7 which follows it will be shown that Items Qthrough I were obtained under charging conditions which give 75 to 100percent of the maximum sustainable charge (peakcharge).

The uniformity of the swath was determined both by swath widthuniformity in the deposited sheet and by visual observation of thenetwork midway between target plate and collecting belt. Uniformity wassatisfactory for ion gun currents of 100 to 325 microamperes. With stillhigher ion gun currents the fiber appeared to be non-uniformlydistributed in the swath. Also under conditions of ion gun currentgreater than 325 microamperes a continuous spark discharge occurredbetween the target plate trailing edge and the fiber which had alreadyleft the edge. This was readily observed by darkening the spinning cell.These observations are recorded in Table II. It is believed that thislightning or spark discharge is responsible for a loss in fiber charge.A discharge of lesser significance occurs at lower ion gun currentsusually in the form of an even glow from the trailing edge of the targetwhen viewed in the dark. All of these discharges from the target plateedge are termed secondary corona.

FIG. 7 is a curve plotted from the data of Table II for items with zerobelt potential.

In FIG. 7, the abscissa indicates the ion gun current as measured by amicroammeter in the negative DC power supply to the ion gun. Threeordinates dimensions are shown on the figure. One of these is beltcurrent. The belt current was the current measured by the microammeter29 between the belt structure and positive DC source 37 as shown in FIG.1 or it was the current measured by microammeter 29 directly to groundwhen the belt was not charged. Other parameters which were derived frombelt current are shown in FIG. 7. These are Web Charge, as determined bythe formula already described and Percent Peak Web Charge. The peak webcharge is identified by Point l2I in FIG. 7. It will be obvious thatPoint 121 indicates not the maximum charge recorded, but the maximumsustainable charge. For operating conditions to the right of Point I themeasured belt current fluctuates when measurements are taken over aperiod of time. It is believed that the fluctuations are due todifferences in web charge which is brought to the belt by theplexifilament material. A high charge is carried to the belt when theamount of lightning discharge is low. Such conditions are represented bythe upper curve marked moderate secondary corona. If a greater amount oflightning discharge occurs while the belt current is being measured thecurrent levels indicated by the lower curve will occur. In practice ofcourse the charge level for conditions to the right of Point 121oscillates randomly between the upper and lower curve. For the purposeof clarity the peak charge is identified as the maximum sustainablecharge as represented by Point 121. The percent of peak charge at anyoperating condition is determined by dividing the given charge by thepeak charge and multiplying the resulting fraction by 100. The chargelevel is calculated from belt current by use of the formulas alreadydescribed.

By comparison of the swath width and standard deviation values of TableII with the curves of FIG. 7, one may determine optimum operatingconditions for the process. In order to achieve maximum swath widthwhile still obtaining low standard deviation in basis weight (Items 40to l2I of Table II) the process of Example I must be operated atconditions represented in FIG. 7 by ion gun currents between 225 and 325microamperes. In this operating range the fiber accumulated a charge of6.75 to 9.00 microcoulombs per gram which is to percent of the maximumsustainable charge for the system operating with belt at groundpotential. I

The process of the invention may be operated with a conductive targetplate having either a non-conductive trailing edge as in the example, ora conductive trailing edge. In operating with a target plate that has aconductive trailing edge, lightning occurs at a lower current level thanfor a plate having an insulated trailing edge.

TABLE II Ion Gun Belt target Sheet weight, ozJyd. Current, Current,lightning Swath micro- Potential, micro- Potential, flashes/ width,Standard Item amperes kilovolts amperes kilovolts min. inches Averagedeviation,

1C 100 20 1 0 0 1.46 .202 150 -23. 5 O 0 17 1.29 .160 200 26. 5 27 0 018 1.15 .142 225 28 30 0 0 19.5 1.10 .142 250 29 33.5 0 0 19.5 1.07 .121275 30.5 36 0 0 20 1.03 I21 300 32 39 0 6 20.5 1.00 12B 300 32 39 0 620.5 .97 115 300 -32.5 39 0 2 20.5 .99 111 300 32 40. 5 +15 20 1.04 I25300 -31.5 40 415 0 20 .98 W 325 33 40 0 21 .96 12:1 350 34 33-41 0 211.02 .169 375 35 29-41 0 13-20 1.10 1 400 35. 5 24-33 0 16. 5-19 1.15425 --36. 5 25-35 0 16-20 1. 2e 200 450 37. 5 25-35 0 16-19 1. 29 240475 38 25-35 0 16-20 1. 23 241 500 40 25-35 0 14-19 1. 31 75 1 Almostcontinuous. 2 Continuous.

Charging curves obtained with a target plate having conductive trailingedge are depicted in FIG. 8. While this target plate is verysatisfactory, the target plate with an insulated rim is preferred, sincea higher peak charge may be obtained. This follows from the fact thatthere is no longer a conductive sharp edge to permit high fieldconcentration between the plexifilament web and the target plate edge orbetween the ion gun and the target plate edge. Higher charge levels arebeneficial for obtaining greater spreading of the network. Of coursewhen operating the target plate with a non-conductive edge, one shouldbe especially careful to operate under conditions on the left side ofthe charging curve depicted in FIG. 7. In this way lightning is avoidedand a higher peak charge is obtained than can be obtained with aconductive edge.

It should be noted that the target plate should be kept clean duringoperation, and efficient conductive paths to ground should bemaintained. The target plate can be provided with a scraper to removedeposits at a point outside of the corona discharge area. The targetplate should have a smooth surface to avoid field concentrations at pitsor points. For example, the surface may be coated with a liquidconditioning material supplied through a wick. Because of the importanceof a clean, smooth surface, the target plate should preferably bepre-conditioned by lapping with an abrasive material such as a 500 gritabrasive cloth before use in the spinning apparatus.

While the present invention has been described with particular referenceto the formation of sheet products solved and the particular extrusionequipment is not critical. Some suggested alternatives may be found inUS. Pat. No. 3,081,519 to Blades and White dated Mar. 19, i963. Whilethe target plate has been described as presenting a flat" surface to thepath of advance of the web and the ion gun, plate curvatures and otherconstructions which do not interfere with the smooth flow of gases andthe mechanically opened web across the target plate may be used. It isimportant for avoiding turbulence as the web leaves the target plate,

that the trailing edge of the plate be flat. While the flat edge isillustrated to be either straight or circular, other edge shapes may beused, provided aerodynamic and electrostatic non-uniformity is avoided.

Mechanical separation of the elements of the web may be accomplished inany manner. A fixed or oscillating baffle is suitable as is a shroudedspinneret device or combination of shroud and baffle. Arrangement of themechanical opening means and target plate in such manner as to preventrecycling vapors from lifting the web away from the plate during itsadvance is particularly desirable. While the system illustrated showsthe imposition of a negative charge on the web while it travels its pathof advance, and a positive charge on the collecting surface, thesepolarities may be reversed.

What is claimed is:

1. A process comprising: flash extruding a solution of organic polymericmaterial intoa gaseous atmosphere to form a plexifilamentary web;spreading the web; passing the spread web through an ionized zonecreated by a corona current between a multi-point ion gun and a groundedtarget electrode to charge the web, said web being passed in brushingcontact with said target electrode, said ion gun being connected to anelectric potential for initiating and maintaining said current;maintaining said current at a level for depositing a charge on said webof from about 75-100 percent of a peak charge, said level being belowthat level for producing said peak charge and depositing the web on amoving collecting surface located below said gun and said electrode.

2. The process of claim 1 wherein said gaseous atmosphere is at leastabout percent gaseous solvent, the remainder being air.

3. The process of claim 2 wherein said gaseous solvent istrichlorofluoromethane.

4. The process of claim 1 including the step of applying a potential onsaid collecting surface which is opposite in polarity to the charge onthe web.

5. The process of claim 1, said current being maintained at a level offrom about 225-325 microamperes.

2. The process of claim 1 wherein said gaseous atmosphere is at leastabout 70 percent gaseous solvent, the remainder being air.
 3. Theprocess of claim 2 wherein said gaseous solvent istrichlorofluoromethane.
 4. The process of claim 1 including the step ofapplying a potential on said collecting surface which is opposite inpolarity to the charge on the web.
 5. The process of claim 1, saidcurrent being maintained at a level of from about 225- 325 microamperes.